1. Field
The present application relates generally to the operation and design of RF systems, and more particularly, to digital to analog convertors.
2. Background
Digital to analog convertors (DACs) are used in a variety of applications, such as in transmitters, to convert digital data into an analog signal for transmission. One type of DAC is referred to as a current steering DAC and operates to convert digital data into a corresponding current that can be used to generate an analog voltage signal representative of the digital data. For example, a current steering DAC switches current at its output based on a digital input so that adjacent current levels differs by an amount corresponding to a least significant bit (LSB). As a result, a 14-bit DAC can output up to 16,384 different current levels.
When a current steering DAC changes state, switching transients (or glitches) are generated which can degrade the DAC's performance. For example, glitches may cause inaccuracies during the conversion of the current output to an analog voltage. Since switching transients are unavoidable, it is desirable that they be identical to minimize their impact on DAC performance.
One way to have identical switching transients is to design the DAC (i.e., 14-bit DAC in this example) with 16,383 individual current sources that are selectively enabled based on the digital code to be converted. Although this solution may assure identical glitches, implementing such a large number of current sources is not very efficient. Another alternative would be to design the DAC with binary weighted current sources. Thus, for a 14-bit DAC, there will be fourteen individually controllable current sources that differ in weight by a factor of two from one source to the next. However, the problem with this solution is that binary weighted current sources can be difficult to produce so that the current matching requirements are accurately met. Additionally, binary weighted current sources will not be able to easily scale the glitches that they generate.
Therefore, it would be desirable to have a DAC that allows switching transients to be controlled so that they scale in the same manner as the switching currents and therefore do not degrade DAC performance.